This invention relates generally to pneumatic impact hammers of the type having an impact piston and a fluid delivery tube received within a coaxial bore in the piston for supplying pneumatic pressure fluid to one or both ends of the piston operating chamber for reciprocation of the piston.
U.S. Pat. No. 5,205,363 discloses a pneumatic impact drill having a porting system for automatically supplying pneumatic pressure fluid from a fluid delivery tube received within a coaxial bore in the impact piston to the opposite ends of the piston operating cylinder as the piston reciprocates. The porting system comprises an annular set of equiangularly spaced outlet ports in the fluid delivery tube and two sets of radially extending bores in the piston having first and second, axially spaced, annular sets of equiangularly spaced inlet ports which cooperate with the outlet ports in the delivery tube. The number of inlet ports in the piston and the number of outlet ports in the supply tube are selected to provide substantial fluid communication therebetween at all relative angular positions of the piston and fluid delivery tube.
Such invention provides a new and improved porting system. However, this porting system uses a single fluid delivery tube port to complete alternative flow paths for the two axially spaced piston ports and requires axial piston movement to initiate transfer from the first flow path to the second flow path. Consequently, pressure conditions that impede the movement of the piston can exist while the fluid delivery tube port is positioned intermediate the two piston ports.
U.S. Pat. No. 5,984,021 discloses a pneumatic impact drill having a porting system for automatically supplying pneumatic pressure fluid from a fluid delivery tube received within an axial bore in the impact piston to the opposite ends of the piston operating cylinder as the piston reciprocates. A flange on the delivery tube is slidably mounted in a cavity in a mounting hub assembly. Passageways provide fluid communication between the pressure fluid inlet plenum and the operating chamber side of the cavity and between the back chamber and the inlet plenum side of the cavity. Fluid pressure forces acting on the opposite faces of the flange and on the top end surface of the delivery tube reciprocate the delivery tube between upstroke and downstroke positions. Positioning the delivery tube in the upstroke position increases the axial distance that the piston must travel before the pressure fluid supply is connected to the back end of the operating chamber on the piston upstroke. Positioning the delivery tube in the downstroke position increases the axial distance the piston must travel before the pressure fluid supply is disconnected from the back end of the operating chamber on the piston downstroke. While the porting system of this invention addresses the deficiencies discussed above, pressure applied to the front chamber of the piston during the downstroke reduce the impact energy produced by the piston.